In recent years, a linear modulation method such as orthogonal frequency division multiplexing (OFDM) has been widely used as a modulation method for improving frequency use efficiency. Signals modulated by the linear modulation method such as OFDM tend to have a high peak to average power ratio (PAPR). Therefore, recently, various amplifiers that amplify a signal having a high PAPR with a high efficiency have been examined. For example, a Doherty amplifier has been known as such an amplifier.
FIG. 3 is a configuration example of a general Doherty amplifier. As depicted in FIG. 3, the Doherty amplifier includes a distributor 11, a carrier amplifier 12, a phase regulator 13, a peak amplifier 14, an impedance converter 15, a synthesizer 16, and an output load 17.
The distributor 11 branches a radio frequency (RF) signal input from an input terminal 11a into two RF signals, and distributes one of the RF signals to the carrier amplifier 12, and the other RF signal to the phase regulator 13. The carrier amplifier 12 amplifies the RF signal input from the distributor 11 according to a voltage applied to a supply terminal 12a, and outputs the amplified RF signal to the impedance converter 15.
The phase regulator 13 regulates the phase of the RF signal input from the distributor 11, and outputs the phase-regulated RF signal to the peak amplifier 14. The peak amplifier 14 amplifies a peak component of the RF signal input from the distributor 11 according to a voltage applied to a supply terminal 14a, and outputs the amplified RF signal to the synthesizer 16. The impedance converter 15 is a transmission line having an electrical length of λ/4, and is connected to an output terminal of the carrier amplifier 12. The synthesizer 16 is connected to an output terminal of the impedance converter 15 and an output terminal of the peak amplifier 14, and synthesizes the RF signal from the impedance converter 15 and the RF signal from the peak amplifier 14. The output load 17 is connected to an output terminal of the synthesizer 16. An impedance value Z0 of the output load 17 is fixed.
In the Doherty amplifier depicted in FIG. 3, it is set such that a voltage Vdd applied to the supply terminal 12a of the carrier amplifier 12 becomes the same as another voltage Vdd applied to the supply terminal 14a of the peak amplifier 14. Furthermore, in the Doherty amplifier, the carrier amplifier 12 operates in the Class AB or Class B, and the peak amplifier 14 operates in the Class B or Class C. That is, only the carrier amplifier 12 operates as a main amplifier until the carrier amplifier 12 becomes saturated, and when the carrier amplifier 12 becomes saturated, the peak amplifier 14 operates as a sub-amplifier.
The efficiency of the Doherty amplifier with respect to an output power varies according to a ratio α between a current I3 output from the impedance converter 15 to the synthesizer 16 and a current I0 output from the synthesizer 16 to the output load 17, when the respective amplifiers operate in the saturated state. Particularly, it is known that the efficiency of the Doherty amplifier becomes favorable when α<0.5.
Therefore, as a conventional technique to realize the ratio of α<0.5, there has been proposed a method of setting a device size of a carrier amplifier different from a device size of a peak amplifier. According to this conventional technique, for example, by setting an emitter area of the carrier amplifier to be smaller than that of the peak amplifier, the device sizes of the carrier amplifier and of the peak amplifier are set different from each other. The related techniques are disclosed in Japanese Laid-open Patent Publication No. 2007-081800, Japanese Laid-open Patent Publication No. 2007-006450, Japanese Laid-open Patent Publication No. 2005-322993, Japanese Laid-open Patent Publication No. 2006-166141, for example.
However, in the conventional technique for setting the device size of the carrier amplifier different from that of the peak amplifier, the impedance of the impedance converter is increased, and its efficiency is degraded as a result.